Average IQ scores have climbed for a century, and nobody fully agrees on why

Across dozens of countries, raw scores on IQ tests rose by roughly three points per decade through most of the 20th century, forcing test-makers to keep re-norming the scales. The pattern is named the Flynn effect after researcher James Flynn, who documented it; whether it reflects nutrition, schooling or just test familiarity is still debated.

— James R. Flynn, What Is Intelligence? Beyond the Flynn Effect — Cambridge University Press, 2007